When buying model railway items such as Bachmann, Heljan or Hornby weathered or renumbered by traders, the important point to look at is the accuracies. Some model railway suppliers simply go and buy locomotives in a casual fashion to weather and renumber and are unaware that they are doing it without checking the accuracies of the actual locomotive's details.
A steam locomotive may be represented by the chimney, tender or British Railways totems while on the diesels, the let down must lie with the factory-fixed four-digit headcodes notably in the green or early blue era as they restrict the choice of a favourite running number or even name depending on where they were allocated. For example, an idea to renumber an Eastern Region Class 47 such as D1500 to D1662 Isambard Kingdom Brunel would produce incorrect results for a Western Region Class 47 to be working with a headcode for a passenger or freight turn on the East Coast Main Line. This was of course abolished by the mid 1970s.
Some diesels incorporate different positions of the running numbers, overall detail differences such as on the Class 25s and their liveries or bodyside grilles and boiler water tanks together with the position of builders plates between Derby and Darlington built batches. But it isn't just the traders that make mistakes but manufacturers do too. Bachmann's D5182 is full of errors as the real D5182 was one of the North Eastern Region batch of early GEC Series 1 Class 25s (D5179-82) that didn't have boiler water tanks and the Darlington builders plate should have been on the lower part of the cab doors. On steam traction, the tenders may differ particularly with the Standard designs. I could go on and on about it but stop just here and hope my points be of help.